A new dual-band planar filter has been proposed. It is shown that the two transmission bands can be excited and designed using proposed resonators which combine different size of open-loop resonators. The main resonators control the low-band resonant frequency and the sub resonators control the high-band resonant frequency. Two passbands can be easily and flexibly designed by adjusting the length of resonators. The proposed dual-band filter will find applications in wireless communication circuits.